linux-kernel - Re: [PATCH v6 1/3] vfs, fdtable: Add get_task

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Message-ID: <20191225161932.fhvbgrcco36mhvaw@wittgenstein>
Date:   Wed, 25 Dec 2019 17:19:33 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     linux-kernel@...r.kernel.org,
        containers@...ts.linux-foundation.org, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, tycho@...ho.ws, jannh@...gle.com,
        cyphar@...har.com, oleg@...hat.com, luto@...capital.net,
        viro@...iv.linux.org.uk, gpascutto@...illa.com,
        ealvarez@...illa.com, fweimer@...hat.com, jld@...illa.com,
        arnd@...db.de
Subject: Re: [PATCH v6 1/3] vfs, fdtable: Add get_task_file helper

On Mon, Dec 23, 2019 at 10:26:46PM +0100, Christian Brauner wrote:
> On Mon, Dec 23, 2019 at 09:08:55PM +0000, Sargun Dhillon wrote:
> > This introduces a function which can be used to fetch a file, given an
> > arbitrary task. As long as the user holds a reference (refcnt) to the
> > task_struct it is safe to call, and will either return NULL on failure,
> > or a pointer to the file, with a refcnt.
> > 
> > Signed-off-by: Sargun Dhillon <sargun@...gun.me>

Could you please add:

Suggested-by: Oleg Nesterov <oleg@...hat.com>

and add a line like:

"This is based on a patch sent by Oleg (cf. [1]) a while ago."

[1]: Link: https://lore.kernel.org/r/20180915160423.GA31461@redhat.com

apart from a few nits below:

Acked-by: Christian Brauner <christian.brauner@...ntu.com>

> > ---
> >  fs/file.c            | 22 ++++++++++++++++++++--
> >  include/linux/file.h |  2 ++
> >  2 files changed, 22 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/file.c b/fs/file.c
> > index 2f4fcf985079..0ceeb046f4f3 100644
> > --- a/fs/file.c
> > +++ b/fs/file.c
> > @@ -706,9 +706,9 @@ void do_close_on_exec(struct files_struct *files)
> >  	spin_unlock(&files->file_lock);
> >  }
> >  
> > -static struct file *__fget(unsigned int fd, fmode_t mask, unsigned int refs)
> > +static struct file *__fget_files(struct files_struct *files, unsigned int fd,
> > +				 fmode_t mask, unsigned int refs)
> >  {
> > -	struct files_struct *files = current->files;
> >  	struct file *file;
> >  
> >  	rcu_read_lock();
> > @@ -729,6 +729,11 @@ static struct file *__fget(unsigned int fd, fmode_t mask, unsigned int refs)
> >  	return file;
> >  }
> >  
> > +static struct file *__fget(unsigned int fd, fmode_t mask, unsigned int refs)

This can be:

static inline struct file *__fget(...)

> > +{
> > +	return __fget_files(current->files, fd, mask, refs);
> > +}
> > +
> >  struct file *fget_many(unsigned int fd, unsigned int refs)
> >  {
> >  	return __fget(fd, FMODE_PATH, refs);
> > @@ -746,6 +751,19 @@ struct file *fget_raw(unsigned int fd)
> >  }
> >  EXPORT_SYMBOL(fget_raw);
> >  
> > +struct file *fget_task(struct task_struct *task, unsigned int fd)
> > +{
> > +	struct file *file = NULL;
> > +
> > +	task_lock(task);
> > +	if (task->files)
> > +		file = __fget_files(task->files, fd, 0, 1);
> > +
> > +	task_unlock(task);
> 
> Nit: remove the \n:
> 
> task_lock(task);
> if (task->files)
> 	file = __fget_files(task->files, fd, 0, 1);
> task_unlock(task);
> 
> Christian